President George W Bush has gone on the electoral offensive with a veiled attack on his Democratic rival for the presidency, John Kerry.
President Bush presented himself as anti-protectionist
In a speech in Virginia, President Bush warned against "economic isolationists" who favoured barriers to free trade.
The attack, which did not mention Mr Kerry by name, follows criticisms of Mr Bush's economic policies by the Democratic presidential challenger.
It came as Mr Kerry took four more states in the latest primary contests.
He won by a landslide in Florida, Louisiana, Mississippi and Mr Bush's home state of Texas.
The Massachusetts senator needs to win one more primary, in Illinois next Tuesday, to secure his party's nomination to become the Democratic presidential candidate at the national convention in July.
Correspondents say Mr Kerry will be looking to send a message to President Bush, in particular with his win in Florida, which is considered to be a key battleground for November.
Mr Bush's attack on Mr Kerry came during a Commerce Department awards ceremony in Arlington.
"As our economy moves forward and new jobs are added, some are questioning whether American companies and American workers are up to the challenge of foreign competition," he said.
"There are economic isolationists in our country who believe we should separate ourselves from the rest of the world by raising up barriers and closing off markets. They're wrong."
White House officials would not say who was the target of Mr Bush's remarks.
However, the president has previously used the phrase "economic isolation" to describe Mr Kerry's policies.
Mr Kerry has criticised Mr Bush for presiding over the loss of three million manufacturing jobs and accused him of leaving a "trail of broken promises".
Correspondents say Mr Bush has himself erected barriers to free trade, notably by imposing tariffs on foreign-made steel which were later abolished after the European Union threatened retaliation.